Cu₂ZnSn(S,Se)₄ (CZTSSe) photovoltaic (PV) technology has attracted much attention due to its cost efficiency, non-toxic nature, and use of earth-abundant elements. However, the current world record power conversion efficiency (PCE) of CZTSSe solar cells is 13.0%, lagging far behind the 23.4% of its predecessor Cu(In,Ga)Se₂ (CIGS). The open-circuit voltage (V_OC) deficit is always the key factor for the unsatisfactory efficiency of CZTSSe solar cells and is mainly related to the mismatch of the energy-band structure and deep-level defects at the interfaces. In this review, the current problems at the interfaces are discussed and recent works on interface engineering are summarized. Finally, we present our views on the challenges and prospects in this field.